1. Field of the Invention
The present invention relates generally to an X-ray computed tomography (CT) apparatus, and more particularly to an X-ray CT apparatus that reduces the X-ray dose applied to a patient.
2. Description of Related Art
In a conventional X-ray CT apparatus, an X-ray tube current supplied to an X-ray source is controlled according to a part of the body of a patient so that an appropriate X-ray dose is applied to the patient.
Japanese Patent Application Laid-open No. 5-305077 discloses a method in which a control pattern for an X-ray tube current at slice locations is determined in advance according to scannograms obtained in two directions. However, the acquisition of the scannograms in the two directions raises the X-ray dose applied to the patient.
U.S. Pat. No. 5,867,555 discloses a method in which an X-ray tube current is controlled according to transmission data that was stored during a last half rotation of a scanner. However, the X-ray tube current is not appropriate when the intervals of the slices are long. In addition, this method is not suitable for some parts of the body of the patient where the characteristics change considerably, such as before and behind the diaphragm.
In view of the forgoing, it is an object of the present invention to provide an X-ray CT apparatus that can raise the signal-to-noise ratio and lower the X-ray dose applied to a patient.
To achieve the above-mentioned object, the present invention is directed to an X-ray CT apparatus, comprising: a first device that produces a two-dimensional model that shows transmission lengths of a patient at rotation angles of an X-ray source from scannogram data obtained by a scannography in a single direction for the patient; a second device that sets X-ray tube currents at the rotation angles of the X-ray source according to the two-dimensional model; a third device that performs a CT scan for the patient by applying the X-ray tube currents to the X-ray source; and a fourth device that reconstructs an image from projection data obtained by the CT scan.
The two-dimensional model is preferably an ellipse.
To achieve the above-mentioned object, the present invention is also directed to an X-ray CT apparatus, comprising: a first device that produces a three-dimensional model that shows transmission lengths of a patient at rotation angles of an X-ray source at slice locations of the patient from scannogram data obtained by a scannography in a single direction at the slice locations of the patient; a second device that sets X-ray tube currents at the rotation angles of the X-ray source at the slice locations according to the three-dimensional model; a third device that performs a CT scan for the patient by applying the X-ray tube currents to the X-ray source; and a fourth device that reconstructs images from projection data obtained by the CT scan.
Preferably, the first device produces a two-dimensional ellipse model at each slice location to produce the three-dimensional model.
To achieve the above-mentioned object, the present invention is also directed to an X-ray CT apparatus, comprising: a first device that produces a three-dimensional model that shows transmission lengths of a patient at rotation angles of an X-ray source at slice locations of the patient from CT projection data obtained by a preliminary helical scanning for the patient with a first X-ray dose; a second device that sets X-ray tube currents at the rotation angles of the X-ray source at the slice locations according to the three-dimensional model; a third device that performs a main helical scanning for the patient with a second X-ray dose that is higher than the first X-ray dose by applying the X-ray tube currents to the X-ray source; and a fourth device that reconstructs images from projection data obtained by the main helical scanning.
Preferably, the model is homogenous and its material is similar to that of the patient""s body, and transmission lengths of the model are converted into those of the patient.